Breath of the Earth: Cycling carbon through terrestrial ecosystems

July 5, 2010, AAAS

This is a view of the tropical forest in Ghana from the top of a flux tower. Credit: Image courtesy of Science/AAAS

Two recent international studies are poised to change the way scientists view the crucial relationship between Earth's climate and the carbon cycle. These reports explore the global photosynthesis and respiration rates -- the planet's deep "breaths" of carbon dioxide, in and out -- and researchers say that the new findings will be used to update and improve upon traditional models that couple together climate and carbon.

The two reports will be published online by the journal Science at the Science Express Web site today.

Christian Beer from the Max Planck Institute for Biogeochemistry in Jena, Germany, along with colleagues from 10 other countries around the world, first take a look at Earth's Gross Primary Production, or GPP, which represents the total amount of carbon dioxide that terrestrial plants breathe in through photosynthesis each year. With a novel combination of observations and modeling, they estimate the total amount of carbon dioxide that the world's plant life inhales annually to be 123 billion tons.

Then, Miguel Mahecha, also from the Max Planck Institute for Biogeochemistry, and another international team of researchers settle a long-standing debate over the effects of short-term variations in air temperature on ecosystem respiration, or the Earth's exhalation of carbon dioxide back into the atmosphere. They show that the sensitivity of ecosystem respiration to short-term variations in temperature is similar around the world. The researchers also suggest that factors other than temperature, such as the slow, ongoing transformations of carbon in the soil and water availability, appear to play crucial roles in long-term ecosystem carbon balances.

Together, these findings shed more light on the global cycle of carbon into and out of the atmosphere and how those processes are coupled with Earth's ever-changing climate. The researchers analyzed vast amounts of climate and carbon data from around the world, and they say their results should help to improve the validity of predictive models and help resolve how climate change might affect the carbon cycle—and our world—in the future.

"An understanding of the factors that control the GPP of various terrestrial ecosystems is important because we humans make use of many ecosystem services, such as wood, fiber, and food," said Beer. "Additionally, such an understanding is important in the context of climate change as a consequence of carbon dioxide emissions from burning fossil fuels because vegetation greatly modulates the land-atmosphere exchanges of greenhouse gases, water, and carbon dioxide…"

In their report, Beer and his colleagues pooled large amounts of data from FLUXNET, an international initiative established more than 10 years ago to monitor exchanges of carbon dioxide between Earth's ecosystems and the atmosphere, with remote sensing and climate data from around the world to calculate the spatial distribution of mean annual GPP between 1998 and 2006.

The researchers highlight the fact that uptake of carbon dioxide is most pronounced in the planet's tropical forests, which are responsible for a full 34 percent of the inhalation of carbon dioxide from the atmosphere. Savannas then account for 26 percent of the global uptake, although the researchers note that savannas also occupy about twice as much surface area as tropical forests.

Precipitation also plays a significant role in determining the gross global carbon dioxide uptake, the researchers found. They suggest that rainfall has a significant influence on the amount of carbon that plants utilize for photosynthesis on more than 40 percent of vegetated lands, a discovery that stresses the importance of water availability for food security. According to the study, climate models often show great variation, and some of them overestimate the influence of rainfall on global carbon dioxide uptake.

"We reached a milestone with this paper by using plenty of data from FLUXNET in addition to remote sensing and climate reanalysis," Beer said. "With our estimation of global GPP, we can do two things—compare our results with [Earth system] process models and further analyze the correlation between GPP and climate."

In the second study, Mahecha and his team of researchers also relied on the global collaboration within the FLUXNET network during their investigation of ecosystems' sensitivity to air temperature. Compiling and analyzing data from 60 different FLUXNET sites, these researchers found that the respiratory sensitivity to temperature of the world's ecosystems, commonly referred to as Q10, is actually quite set in stone—and that the Q10 value is independent of the average local temperature and of the specific ecosystem conditions.

For years, experts have debated the effect that air temperature has on global respiration, or the collective metabolic processes of organisms that return carbon dioxide to the atmosphere from Earth's surface. Most empirical studies suggest that such ecosystem respiration around the world is highly sensitive to increasing temperatures, while the majority of predictive models suggest otherwise. Scientists say that global air temperatures may rise due to the presence of heat-trapping carbon dioxide from the burning of fossil fuels. But, this new result suggests that the temperature sensitivity of the natural exhalation of carbon dioxide from ecosystems has been overestimated and should be reevaluated.

This latest study, in settling the controversy, suggests that previous field studies failed to disentangle processes acting on different time-scales. Mahecha and his team considered the processes of the 60 different ecosystems on the exact same time-scale in order to nail the global mean Q10 down to a value of 1.4. Their new, standard value for various ecosystems' sensitivity to air temperature suggests a less pronounced short-term climate-carbon feedback compared to previous estimates.

"Our key finding is that the short-term temperature sensitivity of ecosystem respiration to air temperature is converging to a single, global value," Mahecha said. "Contrary to previous studies, we show that the sensitivity of ecosystem respiration to temperature variations seems to be independent from external factors and constant across ecosystems. In other words, we found a general relationship between variation in temperature and ecosystem respiration… Our findings reconcile the apparent contradictions of modeling and field studies."

In the future, these two separate studies should allow for more precise predictions of how Earth's warming climate will affect the exchange of carbon between our ecosystems and the atmosphere—and vice versa. They provide scientists with important tools for better understanding the world's ecosystems and how the human race continues to influence and alter them.

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8 comments

There is a very simple, obvious fix to GW which will prevent any runaway greenhouse effect.

It should be obvious that if the ice caps melt so much, then we can plant trees, shrubs, and grasses in siberia, Alaska, Canada, greenland, iceland, and antarctica which will then suck the CO2 out of the air, and eventually reverse the cycle, leading to global cooling.

This one starts to make more sense. The CO2 contribution to global warming or climate change without a precise estimation of the role of the biosphere will always be wrongly determined. Still, CO2 exchange is not the only determinant of global temperatures. I have yet to see studies that correlate climate change with levels of water vapor in the atmosphere. This should be considered specially that it has been observed that there are changes in rainfall patterns associated with warming relating to El Niño as compared to warming that does not relate to El Niño. The amount of snowfall and the extent of flooding following hurricanes and super typhoons should indicate to us that water vapor quantity in the atmosphere should be given due consideration.

First of all, the ocean is the biggest permanent carbon sink. Plants and animals on land have a nearly zero net effect when your talking about permanent sequestration (and that's all the really counts). Most plant carbon gets re-introduced back into the atmosphere through decomposition on a relatively short time scale. Also, the carbon that animals breath out comes from the food they eat which ultimately, at the bottom of the food chain, comes from plants. The net effect of plants and animals on land is so close to zero in most cases that it's not really a factor except on EXTREMELY LONG time scales. Even the carbon that makes it into a river and gets washed out to sea mostly remains unsequestered. Only a small portion ends up settling on the sea floor to become hydrocarbon deposits.

The net warming/cooling impact of plants is much more strongly related reflectivity, not carbon consumption.

Are these people trying to push for saving the rainforests or something?

It should also be noted that nearly all the fluxnet sites are in north america and europe. There are maps of the site locations online. Only sites with really good humidity/pressure measurements can get accurate trace gas measurements, and not all the sites have that either.

With this new data, it will be possible to quantify the increase of CO2 burden of continued biomass loss through deforestation and desertification, for example.

Good that there is now a benchmark, at least, considering that about 50 percent of rainforest(globally) is already gone.

Ultimately, this research points to the fact that human-created loss of biomass, which began at the time that agriculture came into use, has progressed unchecked, with an almost asymptotic increase occurring in the last 250 years or so.

Since there has been little done, globally, to replace that biomass, and with the addition of fossil-fuel combustion, most of the CO2 represented by the biomass and fossil fuel consumption now resides in our atmoshpere, less the portion captured by the oceanic carbon"sink", as noted by GSwift7, above.

"continued biomass loss through deforestation and desertification, for example.

Good that there is now a benchmark, at least, considering that about 50 percent of rainforest(globally) is already gone."

Actually, tropical rainforest has a net carbon balance of nearly zero. It neither consumes nor produces any measurable amount of carbon. It just cycles the same carbon in and out of the atmosphere, without adding or removing anything significant. Cool climate forests and grasslands actually do remove some carbon and sequester it in the ground as compost. However, according to the IPCC, managed comercial logging forests are far more efficient at sequestering carbon than wild forests. Prevention of natural forest fires is key.

If you really want to do something that would make a difference in human carbon added to the atmosphere, then we should go back to using non-biodegradeable containers wherever possible. Plastic is the answer if you look at it that way.

Actually, tropical rainforest has a net carbon balance of nearly zero. It neither consumes nor produces any measurable amount of carbon. It just cycles the same carbon in and out of the atmosphere, without adding or removing anything significant. Cool climate forests and grasslands actually do remove some carbon and sequester it in the ground as compost. However, according to the IPCC, managed comercial logging forests are far more efficient at sequestering carbon than wild forests. Prevention of natural forest fires is key.

@GSwift7- My point was that the CO2 bound up in the ALREADY denuded rain forest/forest/arable land has been added to the ambient side of the cycle, along with fossil-fuel emissions.

If you really want to do something that would make a difference in human carbon added to the atmosphere... Plastic is the answer if you look at it that way.

Why not metal, instead? Far and away more durable, and endlessly recyclable.

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